I've been told that I should run my furnace fan all year round, and by doing this, my A/C will cycle on less in the summer, and so will my furnace in the winter. However, I have two systems (both Carrier units). There is a furnace in my basement, and a furnace for my second floor bedrooms in the attic, (and so is the ductwork).

To me, it seems that if I run the fan all year round upstairs, I will be bringing in hot air from the attic when the A/C isn't running, because the attic is so hot in the summer, thus the A/C will actually be running more in order to keep this area cool. Then in the winter, it seems that that the fan would be doing the opposite by bringing in cold air when the furnace isn't running, thus making the furnace come on more often in order to re-heat the area made cool by the fan being on. So, what's the real scoop about leaving the fan on all year round?

I've also been told that it will help keep my house cleaner. In addition, they tell me the A/C and Heat will cycle on and off less often. This maybe true for my first floor, but my second floor? It just doesn't make sense since I have the ductwork for the bedrooms in the attic, and the furnace is in the attic too.

Leaving the fan on will continuously move air throughout the house. Moving air gives the Home owner the feeling cooling. Moving air does provide a sense of cooling. If your system is sized right and installation is right in the attic it should not be pulling any heat from there. I would rather not run fan all the time and would rely on the cycling of the ac unit to properly dehumidify and cool the house. If unit is cycling correctly you should not notice any difference. If The air balance is correct you should not feel any hotspots either.

You're correct that with the air-handler in the attic you'll have extra heat into the house in the summer and more cold air in the winter. Don't keep the fan on 24/7. Let it run when it's called for by the AC or heat.